OCR · A-Level · Biology
Course version: H420 · Version from Sept 2025
Respiration: (h) the chemiosmotic theory
The chemiosmotic theory explains how ATP is synthesised using a proton gradient across a membrane.
Checked against the OCR H420 specification (opens in a new tab) on .
- Point 8 of 13
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
The chemiosmotic theory explains how ATP is synthesised using a proton gradient across a membrane. During electron transport, energy pumps protons (H⁺) across the inner mitochondrial membrane from the matrix into the intermembrane space, creating an electrochemical gradient. Protons then diffuse back through ATP synthase, a membrane protein that acts as a channel and enzyme. The flow of protons down their gradient provides the energy to phosphorylate ADP, forming ATP. This coupling of electron transport to ATP synthesis via a proton gradient is chemiosmosis. The theory applies to mitochondria and chloroplasts, where membranes separate proton pools.
What earns the marks
- Protons are pumped across the inner mitochondrial membrane, creating a gradient.
- The gradient stores potential energy (electrochemical gradient).
- Protons flow back through ATP synthase down the gradient.
- ATP synthase uses this flow to phosphorylate ADP to ATP.
- The theory links electron transport to ATP synthesis via chemiosmosis.
Common mistakes and how to put them right
Thinking ATP synthase pumps protons.
Put it right: Correct it to protons flowing through ATP synthase.
Ignoring the membrane requirement.
Put it right: Correct it to a proton-impermeable membrane needed to maintain the gradient.
Confusing chemiosmosis with substrate-level phosphorylation.
Put it right: Correct it to gradient-driven ATP synthesis.
Exam tips
- Define the proton gradient and its role clearly.
- Name ATP synthase as both channel and enzyme.
- Explain that the energy comes from the proton gradient, not directly from electrons.
Try the lesson
The start of the Respiration: (h) the chemiosmotic theory lesson, made from checked teaching notes for this specification point.
Have a go first
According to the chemiosmotic theory, what directly drives the synthesis of ATP by ATP synthase?
Listen
The chemiosmotic theory explains how ATP is synthesised using a proton gradient across a membrane. During electron transport, energy pumps protons (H⁺) across the inner mitochondrial membrane from the matrix into the intermembrane space, creating an electrochemical gradient. Protons then diffuse back through ATP synthase, a membrane protein that acts as a channel and enzyme. The flow of protons down their gradient provides the energy to phosphorylate ADP, forming ATP. This coupling of electron transport to ATP synthesis via a proton gradient is chemiosmosis. The theory applies to mitochondria and chloroplasts, where membranes separate proton pools. A strong answer earns marks for these points. Protons are pumped across the inner mitochondrial membrane, creating a gradient. The gradient stores potential energy (electrochemical gradient). Protons flow back through ATP synthase down the gradient. ATP synthase uses this flow to phosphorylate ADP to ATP. Exam tip: Define the proton gradient and its role clearly.
See it
Put it in order
Put these steps in the order they happen.
- 1. Chemiosmosis links electron transport to ATP synthesis
- 2. ATP synthase phosphorylates ADP to ATP
- 3. Proton gradient from matrix to intermembrane space
- 4. Protons flow back through ATP synthase down the gradient
- 5. Electron transport releases energy
- 6. Protons pumped across inner mitochondrial membrane
- 7. Electrochemical gradient stores potential energy
Key words
Say what each word means before you turn the card.
Proton gradient
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Electrochemical gradient
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ATP synthase
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Coupling of electron transport to ATP synthesis
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See every definition
- Proton gradient
- Energy from electron transport pumps protons across the inner mitochondrial membrane from the matrix into the intermembrane space, creating a gradient.
- Electrochemical gradient
- The gradient created across the inner mitochondrial membrane stores potential energy.
- ATP synthase
- A membrane protein that acts as a channel and enzyme, through which protons diffuse back down their gradient.
- Coupling of electron transport to ATP synthesis
- The flow of protons down their gradient provides the energy to phosphorylate ADP, forming ATP, which is chemiosmosis.
Label the diagram
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